- Obligate aerobes
- Bacterial organisms that cannot grow without oxygen.
- Obligate anaerobes
- Bacterial organisms that cannot grow in the presence of oxygen.
- Obligate intracellular parasite
- An organism that can only replicate inside the cell of a host organism.
- Obligate parasite
- Any organism that can only replicate within a host.
- Ocular larva migrans
- An infection of the eye caused by the larva of a parasite that has migrated to that area.
- Ommaya reservoir
- A surgically implanted rubber reservoir placed under the scalp that allows drugs to be injected into the fluid-filled cavity inside the brain.
- Onchocerciasis
- A disease also known as “river blindness,” a chronic infection caused by the roundworm Onchocerca vulvulus and transmitted by blackflies.
- Oncospheres
- Infectious larvae released from the eggs of the pork tapeworm.
- Oocysts
- Some forms of some sporozoan eggs.
- Ophthalmia neonatorum
- A pyogenic infection of the eyes of newborns, caused by the gonorrhea bacterium. It is also known as gonococcal conjunctivitis of the newborn.
- Opportunist
- An organism of the normal flora that takes advantage of a reduction in immune defences, producing another disease.
- Opsonin
- A substance, usually an antibody, that combines with the surface of a microbe to increase phagocytosis.
- Orchitis
- An inflammation of the testes seen in some men who have mumps.
- Organelles
- Subcellular structures that have functions comparable to the organs of Metazoa.
- Orthomyxoviruses
- The influenza viruses from the family Orthomyxoviradae.
- Osteomyelitis
- An infection of the bone that can be either acute or chronic. More than 90% have a bacterial origin, especially by S. aureus, S. pyogenes, and H. influenzae. Acute infections typically involve the growing parts of long bones since blood flow through these areas are sluggish which allows for bacterial buildup and often be seen in children and adolescents who grow rapidly. Chronic osteomyelitis is most often the result of Mycobacterium tuberculosis and secondary to a pulmonary infection. The thoracic region of the vertebrae is the most likely area to be affected first but the hips, knees, and bones of the hands and feet can also be affected. Other bacteria causing chronic osteomyelitis include: Salmonella and other coliform bacteria, Pseudomonas aeruginosa and the spirochaete Treponema pallidum which can cause bone lesions in children suffering from congenital syphilis. In addition, hypersensitivity can cause the formation of gumma on the bones of patients suffering from tertiary syphilis.
- Oxygen radical
- A type of oxygen molecule formed by phagocytes having potent activity against microbes, but also capable of damaging host tissues.
